A high fuel value coal-water slurry which can be injected directly into a furnace as a combustible fuel can supplant large quantities of expensive fuel oil presently being used by utilities, factories, ships, and other commercial enterprises.
For many years, coal-water slurries have been successfully transported long distances by pipeline to point of use, such as a utility. Since practical, cost-effective pipeline slurries do not possess the requisite characteristics for efficient use as fuels, present practice is to dewater, grind the dried coal cake to finer particle sizes, and spray the dried solid particles into the combustion chamber.
Pipeline and fuel coal-water slurries differ markedly in required characteristics because of their different modes of use.
For efficient, low-cost service, slurries which are pumped through pipelines for long distances should have the lowest possible viscosities and rheology which is preferably Newtonian with zero or negligible yield point. In practice, these requirements are achieved by coal concentrations which are considerably smaller than those desired in the fuel slurry. Particle sizes in the upper end of the size distribution range are excessively large for efficient combustion. A typical long-distance pipeline slurry containing no dispersant has a coal concentration of about 40 to 50% and a particle size distribution of 8M.times.0 (U.S. Standard Sieve) with about 20% being -325M.
A great deal of work has been done to make possible higher loadings in pipelinable slurries by adding a suitable organic dispersant which reduces viscosity and improves particle dispersion. A dispersant which has been of particular interest is an anionic compound in which the anion is a high molecular weight organic moiety and the cation is monovalent, e.g., an alkali metal, such as Na or K. The anion attaches to the coal particles to give them a high negative charge or zeta potential, which causes repulsion sufficient to overcome Van der Waal's attraction and, thereby, prevent flocculation with concomitant reduction in viscosity. In accordance with DLVO theory, small monovalent cations maximize the desired negative zeta potential. This phenomenon is discussed in Funk U.S. Pat. No. 4,282,006, which also advises against the use of multivalent cations because they act as counterions which disadvantageously reduce zeta potential. The monovalent salt dispersants have been found to give essentially zero yield points. Pipeline slurries, including those containing the anionic alkali metal organic dispersants, when at rest, tend to separate gravitationally in a short period of time into supernatant and packed sediment which is virtually impossible to redisperse.
For efficient practical use as a fuel, the slurry must have several essential characteristics. It must have long-term static stability so that it can be stored for extended periods of time by suppliers or at the point of use. During such storage, they must remain uniformly dispersed or, at most, be subject to some soft subsidence which can be easily redispersed by stirring. By subsidence is meant a condition in which the particles do not segregate, as in sedimentation, but remain dispersed in the carrier fluid in a gel or gel-like formation. Uniform dispersion is essential for reliably constant heat output. Coal loadings must be sufficiently high, e.g., up to 65 to 70% or higher, to produce adequate fuel value despite the presence of the inert water carrier. The coal particles must be small enough for complete combustion in the combustion chamber. The slurry must also be sufficiently fluid to be pumped to and sprayed into a combustion chamber. However, the low viscosities required for pipelinable slurries are not required for a fuel slurry. Such fuel slurries have eluded the commercial art.
It is obvious that a process which can convert coal directly into a fuel slurry or transform pipeline slurry at its terminal into a fuel slurry having the aforedescribed characteristics without requiring dewatering the coal to dryness would be most advantageous.
Coal-water slurries which have the aforesaid requisite properties for effective use as fuels which can be used as a substitute for fuel oil are disclosed in copending Robert S. Scheffee patent applications Ser. No. 197,853 and 360,523, the teachings of which are hereby incorporated by reference. These applications teach the use of alkaline earth metal organosulfonate dispersants to form stable coal-water fuel slurries which have coal-loading capacity as high as 70% or more and particular bimodal particle size distributions. The divalent metal salt acts both as dispersant and slurry stabilizer. The fuel slurries are thixotropic or Bingham fluids which have yield points; become fluid and pourable under relatively small stresses to overcome the yield point; and have the long-term static stability required for a practical fuel. The viscosities of these slurries, though not excessively large for handling and use, are considerably higher than those obtained with the alkali metal salts.
Fuel slurries, such as those prepared in accordance with the present invention, which have substantially lower viscosities than those obtained with the divalent salts alone, while retaining the same long-term static stability and other properties required for use as a fuel, have important advantages in terms of ease of handling and power consumption.
Generally, the prior art has focused on reducing viscosity and, thereby, increasing loadings and pumpability of pipeline slurries. The art has taught the use of anionic alkali metal and alkaline earth metal organic dispersants as equivalents for these objectives, and has shown the alkali metal dispersants to be superior. None of the references teach or suggest the unique capability of the alkaline earth metal salts as long-term static stabilizers or their combination with alkali metal salt derivatives to produce the stable fuel slurries of the present invention. References of interest include Wiese et al. U.S. Pat. No. 4,304,572 and Cole et al. U.S. Pat. No. 4,104,035 which disclose the use of alkali metal or alkaline earth metal salts of organosulfonic acids to improve slurry loading and pumpability. In both cases the data show the alkali metal salts to be superior for the stated objectives.